She was an adult, and if she is observant about her religion's dietary laws, I would expect her to take the initiative in making sure she doesn't eat something that violates those laws, especially at a neighborhood party where not everyone is of her faith.

I only have a few nodding acquaintances who are Muslims, so I can't speak for that faith, but my Jewish friends have varying degrees of keeping kosher. Some don't keep kosher at all. Some are strict to the point that they will bring their own kosher food to a non-kosher catered affair. Others keep strict kosher at home but are more lenient in public -- they have separate dishes and fridges at home, but they don't care if a restaurant or potluck dish was prepared in a pan that might have held pork or chicken cordon bleu in the past.

If a dietary issue is that critical, it's up to the person with the issue to inquire before eating food prepared by a stranger. OP has nothing to feel guilty about.

I don't think you should've felt bad for unintentionally offending her. Really, what else could you have done? If your mother had assumed her religion and told her which foods she could and couldn't eat, and it turned out she wasn't of that relgion, she might get offended.

If I know someone is allergic to shellfish, or vegetarian, I will point out what they shouldn't have. But I wouldn't think "hmm, that lady kinda looks like she is of the *ABC* religion, I should tell her what to avoid", because that makes assumptions about their appearance, and how strictly they follow their religion etc.

I was thinking about this thread and actually remembered a situation when I went to a friend's house and was about to take a sip of one of the cups which were pre-filled with a mixed drink. Cue a loud scream "NyaChan Wait!!! That has alcohol in it!!" The conversation of the whole group basically stops and everyone is looking at me while I awkwardly explain that yes, the general rule is that alcohol is a no-no for Muslims, but I drink anyways I know she was trying to help, but I wish she had just left me alone. The hostess who was actually a friend of mine unlike this acquaintance hadn't said anything as she knew me well enough to know what I do and don't consume. Instead, it just made me feel guilty and from her reaction to my explanation, I felt like I was being judged.